Over the last years, there has been increasing demand for smaller and thinner semiconductor devices. In this connection, a technique of increasing the packaging density by stacking a plurality of semiconductor devices has also been widely adapted. In view of these demands, there has been developed a technique of forming a feedthrough electrode, which connects the electrode pad formed on the front surface of the semiconductor device to the rear surface through a semiconductor substrate.
For example, Japanese Patent No. 3186941 (published on Aug. 20, 1996) (Patent Document 1) discloses forming a through hole that extends from the rear surface of a semiconductor substrate to the electrode formed on the front surface of the semiconductor substrate, covering the inner wall of the through hole with an insulating film, and then filling the through hole with metal to form a feedthrough electrode. The feedthrough electrode forms a bump that projects from the rear surface of the semiconductor substrate. The publication also discloses a multi-chip module that intends to increase packaging density by stacking a plurality of semiconductor chips having such feedthrough electrodes.
Japanese Laid-Open Patent Publication No. 309221/2003 (Tokukai 2003-309221; published on Oct. 31, 2003) (Patent Document 2) discloses a fabrication method of a BGA (Ball Grid Array) semiconductor device including a feedthrough electrode. In this publication, a through hole is formed that extends from the rear surface of the semiconductor substrate to the electrode formed on the front surface of the semiconductor substrate. Then, after forming an oxide film oh the inner wall of the through hole and the rear surface of the electrode by CVD, anisotropic etching is performed to etch away only the oxide film adhered to the rear surface of the electrode, leaving the oxide film on the side wall. Thereafter, a metal layer is formed inside the through hole, so as to form a feedthrough electrode that connects the front and rear surfaces of the semiconductor substrate.
However, the conventional techniques of forming a feedthrough electrode have problems as described below. Before going into details, the following first describes an exemplary structure of the semiconductor device including a feedthrough electrode, with reference to FIG. 11.
FIG. 11 is a cross sectional view illustrating a structure in the vicinity of an electrode formed in the semiconductor device including a feedthrough electrode. Generally, a first insulating film 102 is formed on a first surface (corresponding to the front surface) of a semiconductor substrate 101, and a multi-layered metal lead layer is formed on the first insulating film 102. The metal lead layer has an electrode pad 103 for sending signals in and out of the semiconductor chip, and the feedthrough electrode is formed in the area where the electrode pad 103 is formed. On the metal lead layer, a protecting film 104 such as an oxide film or a nitride film is formed.
In the semiconductor substrate 101, the through hole is formed directly below the electrode pad 103, and a second insulating film 105 is formed so as to cover the inner wall of the through hole and a second surface (corresponding to the rear surface) of the semiconductor substrate 101. Further, a conductive layer 106 is formed inside the through hole and on the second surface of the semiconductor substrate 101. The conductive layer 106 formed inside the through hole serves as the feedthrough electrode. The second surface of the semiconductor substrate 101 is connected to an external input/output terminal 107, and a protecting film 108 covers the second surface of the semiconductor substrate 101 except for a portion where the external input/output terminal 107 is formed. In this way, the conductive layer 106 connects the electrode pad 103, formed on the first surface of the semiconductor substrate 101, to the external input/output terminal 107, formed on the second surface of the semiconductor substrate 101.
In fabricating the semiconductor device of the structure shown in FIG. 11, the second insulating film 105 is formed, for example, by a CVD (Chemical Vapor Deposition) method from the second surface side of the semiconductor substrate 101 having been formed with the first insulating film 102, the electrode pad 103, and the protecting film 104.
However, in this case, the second insulating film 105 is undesirably formed on the rear surface of the electrode pad 103 which needs to be conducted to the feedthrough electrode, as shown in FIG. 12(a). Therefore, before forming the conductive layer 106, it is required to remove the second insulating film 105 formed on the rear surface of the electrode pad 103, without removing the second insulating film 105 formed on the inner wall of the through hole, as shown in FIG. 12(b). There are several techniques of removing the second insulating film 105 formed on the rear surface of the electrode pad 103.
In the first technique, a resist is applied on the rear surface of the semiconductor substrate, and the resist inside the through hole is removed by photolithography. The insulating film on the rear surface of the electrode pad is then etched away by dry etching.
The second technique employs anisotropic dry etching, whereby the insulating film on the rear surface of the electrode is etched without etching the insulating film formed on the side wall of the through hole. The foregoing Patent Document 1 employs this technique.
A problem of the first technique is the difficulty in uniformly filling the through hole with the resist when the resist is uniformly applied over the rear surface of the semiconductor substrate formed with the though hole. With finer feedthrough electrodes, it becomes extremely difficult to fill the through hole with the resist and remove the resist inside the through hole by development.
As a rule, the electrode used for the semiconductor device is no greater than 100 μm2. When a Si wafer is used as the semiconductor substrate for example, the wafer is generally used in a thickness of 100 μm to 700 μm. For example, with a 100 μm thick Si wafer with 70 μm2 through holes, the resist cannot be easily applied uniformly inside the through holes. With finer electrodes as small as 10 μm in diameter and 50 μm in depth, the difficulty multiplies.
Even when it is possible to uniformly fill the through holes of such minute dimensions with the resist, it is still difficult to remove the resist by development because, with such an aspect ratio, the developer has a trouble circulating inside the through holes.
With the second technique, the insulating film on the rear surface of the electrode pad can be removed more easily compared with the first technique.
However, with the second insulating film formed by depositing an oxide film inside the through hole by CVD, the thickness of the insulating film becomes thinner on the inner wall of the through hole than on the rear surface of the semiconductor substrate. Further, in etching the insulating film on the rear surface of the electrode pad by anisotropic etching, the insulating film on the rear surface of the semiconductor substrate is also etched because the etching rate is faster for the insulating film on the rear surface of the semiconductor substrate than for the insulating film on the rear surface of the electrode pad at the bottom of the through hole. Further, despite that the etching is anisotropic, some reduction of the insulating film on the inner wall of the through hole cannot be avoided.
Another drawback of the second technique is that the through hole formed in the semiconductor substrate needs to be tapered so as to more easily perform the post steps of the second insulating film etching, i.e., the step of forming a metal film for forming a conductor in the through hole, or the step of filling the through hole with the conductor. The tapered through hole causes more etching reduction of the second insulating film on the inner wall of the through hole.
The problem of the second technique, then, is that it cannot provide enough thickness for the second insulating film formed on the inner wall of the through hole, or in some cases completely removes the second insulating film. That is, reliability of the feedthrough electrode suffers.
The reliability problem caused by the thickness reduction of the insulating film on the rear surface of the semiconductor substrate can be avoided by forming an insulating film of a suitable thickness on the rear surface of the semiconductor substrate in advance, or by forming another insulating film on the rear surface of the semiconductor substrate after the insulating film on the rear surface of the electrode pad has been etched away. However, both of these techniques increase fabrication cost. Further, the techniques are not effective for the reliability problem caused by the thickness reduction of the second insulating film formed on the inner wall of the through hole.